The present invention relates to a method for detecting a full charge state of a battery during a charging operation, a device employing the method, and a cellular telephone employing the method.
Since a secondary battery (hereinafter simply referred to as a battery) is reusable by recharging, it is widely used as a power supply for a portable device such as a cellular telephone. However, overcharging damages a battery to cause reduction in its capacity and shortening of its useful life. Therefore, overcharging is generally prevented by continuous monitoring of a battery voltage during a charging operation. To put it concretely, since a battery voltage lowers slightly after the battery has reached a full charge state, it is a common practice to detect the full charge state of the battery by detecting the slight lowering -.DELTA.V in the battery voltage during the charging operation.
An example of a conventional method for detecting a full charge state of a battery will be described below.
Initially, a battery voltage is measured by, for example, a measuring circuit (see FIG. 6), and is then converted into digital data by an A/D converter (not shown in the figure) of the measuring circuit. To assure the accuracy, the battery voltage is measured a plurality of times to obtain a plurality of sampling data items, and a mean value of the sampling data items is used as evaluation data for the battery voltage. A detection of a full charge state of the battery is rendered on the basis of whether or not the evaluation data (or the mean value) has reached a predetermined reference value.
In the above-mentioned conventional method, it is necessary to obtain an accurate battery voltage in order to accurately determine whether or not the battery has reached the full charge state. However, the A/D converter which can be mounted on, for example, a cellular telephone to enable the detection of the full charge state has a limited resolution, which resulted in a significant error in A/D conversion process in actuality. Further, the battery which is being charged exhibits a fluctuation in the battery voltage in accordance with a variation in power dissipation of the cellular telephone. The conventional practice of sampling the battery voltage a plurality of times to derive the mean value which is then used as the evaluation data is intended to accommodate for such problem.
Nevertheless, during the actual operation of the cellular telephone as a portable device, the power dissipation of the cellular telephone frequently undergoes a sporadic sudden change, which cannot be fully accommodated for by the above-described data mean technique. The known way to accommodate for this situation is to increase the number of the sampling data items which are used in determining the mean value. However, under the current status of the art, it is difficult to provide a satisfactory result of the detection of the full charge state even if the number of the sampling data items is increased. Further, when the number of the sampling data items is increased, the amount of data processing required for the detection of the full charge state is voluminous, increasing a load on the cellular telephone. By way of example, a cellular telephone of US-CDMA type involves a greater variation in power dissipation in comparison to a cellular telephone of US-TDMA type, and requires, in one instance, that a measurement of the battery voltage is conducted two hundred times in order to render a single determination. The amount of data processing required for the cellular telephone of US-CDMA type is as much as nearly twice that required for the cellular telephone of US-TDMA type.